FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

The fourth lesson learned was that there is a practical limit to an inspector's
ability to interpret indications and that there should be no "loss of face"
involved in requesting a tube to be removed to diagnose an unusual indication.
As a demonstration that an inspector can be tempted to extrapolate theoretical
experience too fer, we give the following example:
At another plant in the examination of a conaenser tube which had been in
service for many years, our inspector attributed a particular type of signal to
a "magnetic" corrosion film which he felt must be on the inner surface of the
tube (Figure 3). He was even able to demonstrate how this signal could be
generated by magnetic tape wrapped around the inside of the tube (Figure 4).
the signal was of completely the wrong phase angle to be interpreted as either
an internal or an external defect. Metallographie examination of a tube pulled
from the condenser a year later, failed to reveal this "magnetic film". It
did, however reveal numerous corrosion pits on the internal tube surface
(Figure 5). It was then demonstrated that the combined effect of closely
spaced pits could rotate the angle of the resultant signal to such a degree
that it would be unrecognizable as a partial through-wall defect. Five years
ago when this incident occurred, the effect of multiple defects was not
reported in the literature. Some research is now being carried out to
elucidate these effects.
The fifth and final lesson learned was that certain problems were beyond the
capability of conventional eddy current techniques and that research laboratory
back-up was essential.
As a last illustration, I would like to talk about the detection of
circumferential cracks in titanium condenser tubes at Pt. Lepreau G.S. All
eddy current inspectors know that conventional bobbin probes used for tube
inspections are very insensitive to circumferential cracks. When it was
discovered that flow induced vibration had caused fatigue failure of some outer
condenser tubes at Pt. Lepreau, we were faced with the problem of identifying
within a few hours, a sufficiently sensitive method to detect other cracked
tubes before they became major leakers. The solution was a new technique
called "3D"*.
This novel technique is so called because of its ability to add another
dimension to eddy current testing — in addition to defect depth and volume,
this equipment can also be used to determine its anqular position in the tube
Wäll.
A "3D" eddy current instrument is similar to a conventional eddy current
instrument except that the 3D generates a three phase sine wave source. The
special 3D probe is tx three coil probe connected in a three phase star
configuration.
* Developed and manufactured by Eddy Current Technology.